Maximum pacing rate limiter system

ABSTRACT

A maximum pacing rate limiter for use in adaptive rate pacing in conjunction with a cardiac rhythm management system for a heart. The maximum pacing rate limiter may function to measure an interval, termed the ERT interval, between a paced ventricular evoked response and a T-wave. The maximum pacing rate limiter may further function to maintain the ERT interval at less than a certain percentage of the total cardiac cycle. In one disclosed embodiment, a maximum pacing rate limiter calculates an ERT rate based on the detected paced ventricular evoked response and the T-wave, and the pacing rate limiter module further communicates the minimum of the ERT rate and an adaptive-rate sensor indicated rate to a pacemaker.

RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 11/225,712 filed on Sep. 13, 2005 and entitled “Maximum Pacing RateLimiter Implemented Using the Evoked Response—T-Wave Interval, which isa divisional of U.S. patent application Ser. No. 10/053,223 filed onJan. 17, 2002 and entitled “Maximum Pacing Rate Limiter ImplementedUsing the Evoked Response—T-Wave Interval,” now issued as U.S. Pat. No.6,963,778, the entirety of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to cardiac rhythm management systems for theheart. In addition, the invention relates to a maximum pacing ratelimiter implemented using the evoked response—T-wave interval.

BACKGROUND

Cardiac rhythm management (CRM) systems are a common solution forproblems associated with the heart's inherent pacing capabilities. Thefundamental components of a CRM system may include a pacemaker forcreating electrical pulses to stimulate the heart and one or moreelectrodes for delivering the electrical pulses and sensing the heart'scontraction in reaction to the stimulus. The heart's contraction inresponse to the electrical pulse is termed the evoked response.

Typically, the CRM system may monitor the heart for a set escapeinterval. An escape interval is a period of time during which thepacemaker will wait to send another electrical pulse to the heart. Ifthis escape interval is exhausted without detection of a natural heartcontraction, an electrical pulse may be delivered to the heart.Electrical pulses may be delivered at a set pacing stimulation frequencybased on the duration of the escape interval.

Pacing abnormalities may occur in both the atrial and ventricularportions of the heart. For heart abnormalities, including such examplesas total or partial heart block, arrhythmias, myocardial infarctions,congestive heart failure, congenital heart disorders, and various otherproblems, a pacing system for the ventricles may include one or moreadaptive rate sensors. An adaptive rate sensor is a sensor that mayfunction to monitor an individual's physical activity. If the adaptiverate sensor determines that additional cardiac output is desirable forthe physiological requirements of an activity, the adaptive rate sensormay increase an adaptive-rate sensor indicated rate. The adaptive-ratesensor indicated rate is a rate provided by an adaptive rate sensor thatmay be used to decrease the duration of the escape interval of apacemaker so that pacing stimulation frequency increases. As the pacingstimulation frequency is increased and the escape interval decreased,the heart is caused to beat at an increased rate and cardiac output maythereby be increased.

A prior art CRM system 100, including an adaptive rate sensor 110coupled to a CRM device 130, is shown in FIG. 1. The adaptive ratesensor 110 may include an adaptive rate sensor or other such device orsignal that functions to provide an adaptive-rate sensor indicated rate.The adaptive rate sensor 110 may communicate the adaptive-rate sensorindicated rate to the CRM device 130. The CRM device 130 may be apacemaker or other such device that functions to pace the heart. The CRMdevice 130 may calculate an escape interval based on the adaptive-ratesensor indicated rate and deliver electrical pulses to the heart basedon the escape interval.

Adaptive rate sensors may be used to detect a wide range ofcardiac-output requirements and increase the pacing stimulationfrequency according to the increased activity of the individual.Examples of activities that may necessitate an increase in pacingstimulation frequency are strenuous activities such as jogging orswimming, although any increase in activity may heighten an individual'scardiac output needs.

Presently there are three major types of commercial adaptive ratesensors available, including activity sensors, minute ventilationsensors, and QT interval sensors. All three types of sensors usedifferent physiological criteria to measure changes in activity andtherefore increased need for cardiac output. Unfortunately, all threesensor types may exhibit limitations in their use under variouscircumstances.

An activity sensor measures the acceleration of an individual bytypically using an accelerometer. In situations such as, for example,when an individual goes from standing still to walking, the individual'sneed for cardiac output increases. The activity sensor measures anincrease in acceleration as the individual walks and therefore increasesthe pacing stimulation frequency for the heart. This increase in thefrequency of stimulation increases cardiac output. A limitation of thistype of sensor is that the increase in pacing stimulation frequency isnot always proportional to the increased workload for the individual,such as when the individual remains relatively stationary (and thereforethe sensor records no change in acceleration) but experiences aheightened increase in the need for cardiac output. Examples of thissituation include riding a stationary bike, where no accelerationoccurs, and to a lesser extent, walking up stairs, where workload is notproportional to acceleration.

A minute ventilation sensor functions to monitor the breathing of theindividual and equates increased respiration with the need for increasedcardiac output. Specifically, the minute ventilation sensor can measurethe volume of air inhaled and exhaled during a particular period oftime, typically by measuring transthoracic impedance. An example of sucha minute ventilation device can be found in U.S. Pat. No. 6,161,042 toHartley et al. If the impedance-based minute ventilation sensor detectsan increase in respiration, it assumes that there is an increase in theneed for cardiac output and the sensor therefore increases the pacingstimulation frequency. While this type of sensor functions moreproportionally to workload, a limitation of this sensor is thesignificant variation among individuals requiring individualizedcalibration. Another limitation of the minute ventilation sensorincludes motion artifact, a phenomenon wherein certain movements by anindividual, such as waving the individual's arms in the air, may beincorrectly interpreted by the impedance-based sensor as an increase inrespiration.

A QT interval sensor functions to measure the interval between stimulusof the ventricle (Q) and the appearance of the T-wave signifyingrepolarization of the ventricle, as indicated on a typicalelectrocardiogram. A shortening of the QT interval may represent anincrease in the need for cardiac output. Once again, a limitation ofthis type of sensor is a variation in QT intervals from individual toindividual. For example, some individuals exhibit a QT interval thatactually lengthens in duration during situations in which increasedcardiac output is desirable. In addition, the QT interval sensor detectsthe interval from stimulation of the ventricle Q to detection of theT-wave, but contraction of the ventricle muscle actually occursapproximately 50 ms after stimulus. Therefore, in the relatively smallQT interval of 120-250 ms, this lag between stimulus and contraction canhave a significant impact on the accuracy of the measured rate ofcontraction.

A shortcoming common to all adaptive rate sensors is the failure toprovide meaningful limitation on pacing stimulation frequency at highpacing rates. This failure to limit the pacing rate can lead tosituations in which the pacing stimulation frequency exceeds the cardiacoutput needs of the individual. More importantly, the increase of pacingstimulation frequency beyond a certain threshold can actually result ina decrease, rather than increase, in cardiac output. This phenomenon mayoccur because the ventricle may be caused to contract at a rate that isfaster than the ventricle can fill with blood.

Regarding activity sensors, because the rate provided by an activitysensor is not proportional to workload, there is no feedback provided toallow the activity sensor to immediately determine if theactivity-sensor indicated rate exceeds the needs of the individual.Therefore, because no negative feedback exists, it is possible at highpacing stimulation frequencies for the activity sensor to provideadaptive-rate sensor indicated rates that exceed the individual'scardiac output needs.

A minute ventilation sensor may be proportional to workload and mayexhibit some negative feedback characteristics. However, if a minuteventilation sensor is not calibrated appropriately, it too can indicaterates that exceed the needs of the individual, particularly if motionartifact is introduced. Further, motion artifact limitations are stillpresented in the minute ventilation sensor.

QT interval sensors do not have negative feedback in their normaloperating range for adaptive-rate pacing. In fact, a QT interval sensordemonstrates a positive feedback characteristic because of themethodology utilized by the QT interval sensor to measure increasedcardiac output need. This positive feedback is illustrated as follows:

(a) the QT interval sensor detects a decrease in the duration of theindividual's QT interval and therefore increases the adaptive-ratesensor indicated rate;

(b) the increased adaptive-rate sensor indicated rate causes thepacemaker to decrease the duration of the escape interval, therebycausing an increase in the pacing stimulation frequency;

(c) the increase in pacing stimulation frequency causes the duration ofthe individual's QT interval to decrease; and

(d) the QT sensor detects this additional decrease in the duration ofthe individual's QT interval and therefore further increases theadaptive-rate sensor indicated rate.

Therefore, positive feedback may occur in this situation where anincrease in pacing stimulation frequency can cause a decrease in theduration of an individual's QT interval, thereby causing the QT intervalsensor to further increase the adaptive-rate sensor indicated rate.

The most sophisticated CRM systems implement a pair of the adaptive ratesensors described above (commonly activity and either minute ventilationor QT interval) to emphasize the strengths of particular sensors andlessen the limitations associated with each particular individualsensor. Such a system is the PULSAR™ MAX CRM system manufactured byGuidant Corporation, which utilizes an activity sensor in conjunctionwith a minute ventilation sensor. However, even in combination, thecurrent adaptive rate sensors may have limitations, and therefore thepotential exists for increased pacing stimulation frequencies thatexceed an individual's cardiac output needs and may actually result indecreased cardiac output.

The adaptive-rate sensor indicated rate is defined herein to furtherinclude an intrinsic atrial rate as measured from the atria of theheart. An intrinsic atrial rate is typically utilized as a method ofpacing the heart in individuals where natural conduction pathwaysbetween the atria and ventricles have been damaged, such as in leftbranch bundle block. In cases such as these, an electrode or other suchsensor is placed in the atria to detect atrial contraction andcommunicate this contraction to the ventricles via a pacemaker or otherdevice.

A common problem that may occur with use of intrinsic atrial pacing isatrial tachycardia, in which an individual's intrinsic atrial pacingrate reaches irregularly high rates. If the intrinsic atrial rateexceeds a certain threshold, use of the intrinsic atrial rate to pacethe ventricles can cause the ventricles to contract at excessive rates.Therefore, use of the intrinsic atrial rate as a method of pacing theventricles may also result in pacing stimulation frequencies that exceedan individual's cardiac output needs and may actually result indecreased cardiac output.

SUMMARY

Generally, the present invention relates to cardiac rhythm managementsystems for the heart, and more particularly, to a maximum pacing ratelimiter implemented using the evoked response—T-wave interval. In oneaspect of the disclosure, a method for limiting a paced heart rate cancomprise measuring an ERT interval, the ERT interval spanning detectionof a paced ventricular evoked response and detection of a T-wave;calculating a maximum pacing rate based on the ERT interval; andadjusting the paced heart rate based on the maximum pacing rate.

In another aspect of the disclosure, a pacing rate limiter for limitinga paced heart rate may comprise an ER sensor adapted to detect a pacedventricular evoked response and a T sensor adapted to detect a T-wave,wherein the pacing rate limiter calculates an ERT rate based on thepaced ventricular evoked response and the T-wave.

In yet another aspect of the disclosure, a cardiac rhythm managementsystem for a heart may comprise a pacemaker module coupled to the heartso as to provide a paced heart rate; a pacing rate limiter modulecoupled to the pacemaker module; a paced ventricular evoked responsesensor module coupled to the pacing rate limiter module, wherein thepaced ventricular evoked response sensor module is adapted to detect apaced ventricular evoked response; a T sensor module coupled to thepacing rate limiter module, wherein the T sensor module is adapted todetect a T-wave; and an adaptive rate sensor module coupled to thepacing rate limiter module, wherein the adaptive rate sensor modulecommunicates an adaptive-rate sensor indicated rate to the pacing ratelimiter module. The pacing rate limiter module calculates an ERT ratebased on the detected paced ventricular evoked response and the T-wave,and the pacing rate limiter module further communicates the minimum ofthe ERT rate and the adaptive-rate sensor indicated rate to thepacemaker module.

In yet another aspect of the disclosure, a pacing rate limiter forlimiting a paced heart rate may comprise a means for detecting a pacedventricular evoked response; a means for detecting a T-wave; and a meansfor calculating a maximum pacing rate based on the paced ventricularevoked response and the T-wave.

In another aspect of the disclosure, a method for calculating a maximumpacing rate may comprise measuring a cardiac cycle, wherein the cardiaccycle comprises a ventricular relaxation portion and a ventricularcontraction portion and limiting the maximum pacing rate to cause theventricular relaxation portion to be a percentage of the cardiac cycle.

The above summary of the present invention is not intended to describeeach disclosed embodiment or every implementation of the presentinvention. The figures and the detailed description which follow moreparticularly exemplify these embodiments.

DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 illustrates a prior art cardiac rhythm management system.

FIG. 2A is an electrocardiogram waveform of a typical heart.

FIG. 2B is a waveform of a typical heart as measured internally using anintra-cardial or subcutaneous electrode.

FIG. 2C is a timeline illustrating relaxation and contraction for theventricles of a typical heart.

FIG. 3 shows the operational flow of a pacing rate limiter according toan embodiment of the present invention.

FIG. 4 illustrates a cardiac rhythm management system including a pacingrate limiter according to an embodiment of the present invention.

FIG. 5 illustrates in more detail a pacing rate limiter according to anembodiment of the present invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention.

DETAILED DESCRIPTION

The present invention is believed to be applicable to cardiac rhythmmanagement (CRM) systems for the heart. In particular, the presentinvention is directed to a maximum pacing rate limiter implemented usingthe evoked response—T-wave interval. While the present invention is notso limited, an appreciation of various aspects of the invention will begained through a discussion of the examples provided below.

In a typical CRM system, a pacemaker that is coupled to the heart iscontrolled using one or more adaptive rate sensors. As described above,an adaptive rate sensor functions to monitor the activity of anindividual and increase pacing stimulation frequency when physiologicalneeds of the individual require increased cardiac output. Examples ofdifferent adaptive rate sensors, as described above, include activity,minute ventilation, and QT interval adaptive rate sensors. An adaptiverate sensor may function to communicate an adaptive-rate sensorindicated rate to a pacemaker. The adaptive-rate sensor indicated rateis a rate provided by an adaptive rate sensor that may be used to adjustthe escape interval of a pacemaker so that pacing stimulation frequencyincreases. The adaptive-rate sensor indicated rate is herein defined toalso include an intrinsic atrial rate as measured for individualssuffering from abnormalities such as left bundle branch block or othersimilar problems. Typically, the pacemaker may increase the pacingstimulation frequency based on the adaptive-rate sensor indicated rate.

A pacing rate limiter according to a preferred embodiment of the presentinvention may act as an upper limit, or governor, to the adaptive-ratesensor indicated rate. A pacing rate limiter according to a preferredembodiment of the present invention may provide negative feedback,thereby assuring that the pacing stimulation frequency does not exceedthe needs of an individual or result in decrease in a cardiac outputbecause of excessive pacing stimulation frequencies.

The methodology or manner by which a pacing rate limiter according to apreferred embodiment of the present invention may function requires anexamination of the physiology of the heart. FIGS. 2A-2C are providedaccording to an identical scale on an X-axis, the X-axis representing aninterval of time. The onset of the interval is marked as 201 and theconclusion is marked as 299. The interval 201-299 may function torepresent a single cardiac cycle for a heart.

In FIG. 2A, a typical electrocardiogram (ECG) waveform is provided. Asin other ECGs, P represents atrial depolarization, the QRS complexrepresents ventricular depolarization, and the T-waveform representsventricular repolarization.

The waveforms as shown in FIG. 2B illustrate measurements taken of atypical heart as measured internally using an intra-cardial orsubcutaneous electrode. An electrical pulse 250 is communicated to theheart by a pacemaker or other such device in order to initiatecontraction of the ventricles, and waveform ER represents the pacedventricle evoked response, with onset at 255 and conclusion at 280. TheER waveform is the actual response by the ventricular muscle to theelectrical pulse 250. The interval between the electrical pulse 250 andthe onset of the evoked response 255 is typically approximately 50milliseconds. This interval 250-255 represents the delay betweenstimulation of the ventricle and actual onset of contraction of theventricle. Methods to detect the paced ventricular evoked response, suchas through use of small coupling capacitor technology, are now known andare discussed generally in U.S. Pat. No. 5,941,903 to Zhu et al. andU.S. Pat. No. 6,067,472 to Vonk et al., both of which are incorporatedherein by reference.

In FIG. 2C, the interval 201-299 on the X-axis once again represents asingle cardiac cycle, and intervals R and C represent relaxation andcontraction of the ventricles, respectively. During interval R, theventricles are in a relaxed state, and blood is allowed to fill theventricles. During interval C, the ventricles are contracting orcontracted, and blood is forced from the ventricles.

As FIGS. 2A-2C are all drawn to approximately the same scale on theX-axis, it can be seen that the interval of contraction of theventricles C can be approximated by the interval QT in FIG. 2A. However,the interval QT fails to account for the lag between the electricalpulse 250 by which the ventricle is stimulated and actual onset ofcontraction of the ventricles 255.

Therefore, a better approximation of the interval of contraction isshown in FIG. 2B as the interval from the paced ventricular evokedresponse ER to the T-waveform as shown in FIG. 2A. This interval betweenthe evoked response ER and the T-waveform is herein defined as the ERTinterval. Specifically, the ERT interval is the interval betweendetection of a specified segment of the waveform representing the evokedresponse ER and detection of a specified segment of the T-waveform.Detection of a specified segment of the evoked response ER andT-waveforms may include onset detection, peak detection, conclusiondetection, or may include detection of a different segment of therelevant waveform.

The ERT interval may provide a more precise approximation of theinterval C representing the contraction of the ventricles. Measuring twoor more ERT intervals and averaging the intervals using known methods,such as for example ensemble averaging, may further refine accuratemeasurement of the ERT interval.

During typical periods of low activity, the interval R representsapproximately 65-75% of the cardiac cycle 201-299 and interval Crepresents approximately 25-35% of the cardiac cycle. During periodsinvolving increased cardiac output, the contraction interval C decreasesin duration as the heart speeds up such that its percentage of thecardiac cycle remains relatively constant. In other words, although thetotal cardiac cycle, represented by the interval between 201 and 299,may decrease in duration as the pacing stimulation frequency increasesand the heart beats faster, the relative percentages that R and C occupyin the cardiac cycle remain relatively constant, or approximately 65-75%and 25-35%, respectively.

Eventually, however, the pacing stimulation frequency reaches a limitsuch that further increase in stimulation frequency can increase heartrate but the speed of ventricular contraction cannot be furtherincreased. Thus, the contraction interval C becomes a larger proportionof the total cardiac cycle 201-299. This necessarily means that therelaxation interval R becomes a smaller proportion of the total cardiaccycle. Because of this phenomenon, there is less of the cardiac cycledevoted to diastolic filling of the ventricles as represented by R. Asthe rate of the stimulation frequency further increases, the ventricleshave less and less time during relaxation interval R to fill with blood,and at a critical point cardiac output may actually decrease because toomuch of the total cardiac cycle 201-299 is devoted to contractionwithout enough time being devoted to relaxation to allow for filling ofthe ventricles.

It is therefore desirable to ensure that the pacing stimulationfrequency is not increased to the point that the contraction interval Coccupies too much of the cardiac cycle. As an accurate approximation ofinterval C, the ERT interval can be measured and compared to the totalcardiac cycle to determine the relative percentage of the cardiac cycledevoted to contraction. It is desirable, therefore, to limit the ERTinterval as shown in Equation 1:

ERT interval<Y% (cardiac cycle)  (1)

In Equation 1, the ERT interval is maintained as no greater than apercentage “Y” of the cardiac cycle.

If the ERT interval is longer than a given percentage “Y” of the cardiaccycle, the pacing stimulation frequency can be limited or decreased. Ina preferred embodiment of the invention, the percentage Y is 50%, suchthat the ERT interval is limited so as never to be greater than 50% ofthe total cardiac cycle. Other percentages for Y may also be selected.For example, the percentage Y may vary as between approximately 30-60%so as to maximize hemodynamic efficiency. In this manner, the pacingstimulation frequency may be limited such that it never reaches thecritical point at which the contraction interval C occupies too great apercentage of the cardiac cycle and therefore cardiac output may bedecreased.

In the situation where an artificial stimulus such as a pacemaker isused to pace the heart, an escape interval represents the outer boundsfor the length of the cardiac cycle. The escape interval is the periodof time that a pacemaker will wait before sending an electrical pulse tothe heart. If the pacemaker does not detect a contraction before theconclusion of the escape interval, the pacemaker will send an electricalpulse to the heart. As the escape interval decreases, the pacingstimulation frequency necessarily increases. If the ERT intervaloccupies more than a certain percentage of the escape interval, becausethe escape interval is too short in duration (and the pacing stimulationtoo fast), a decrease in cardiac output may result. Therefore, it wouldbe desirable to maintain the ERT interval at some percentage of theescape interval as shown in Equation 2:

escape interval>k×(ERT interval)  (2)

As this Equation 2 illustrates, the ERT interval, as an approximation ofthe time the ventricles are contracting, may preferably be maintained asa factor k less than the escape interval. The constant k may be linearor nonlinear, and in a preferred embodiment of the invention may be inthe range of 1.8-2.2. The constant k may also vary depending on suchfactors as how the evoked response ER and T-wave are measured or mayvary depending on the particular needs of the individual. If theconstant k is set equal to 2, then Equation 2 is identicalmathematically to Equation 1 because the ERT interval is maintained atno greater than 50% of the cardiac cycle, or in this case the escapeinterval.

Application of the above principles may be employed by a pacing ratelimiter according to a preferred embodiment of the invention asillustrated by the methodology 300 shown in FIG. 3. In module 310, apacing rate limiter detects the paced ventricular evoked response (ER).In module 320, the pacing rate limiter detects the T-wave. The detectionin modules 310 and 320 may include detection of the onset, peak,conclusion, or other segment of the waveform depending on the detectionmethod used. In module 330, an ERT interval is calculated as theinterval of time between the ER and T-wave. Module 330 furthercalculates an ERT rate (in seconds) based on the ERT interval as followsin Equation 3:

$\begin{matrix}{{{ERT}\mspace{14mu} {rate}} = \frac{60}{\left( {k \times {ERT}\mspace{14mu} {interval}} \right)}} & (3)\end{matrix}$

The ERT rate is calculated in Equation 3 by dividing 60 seconds by theproduct of the constant k, as described in Equation 2, and the ERTinterval (measured in seconds). The ERT rate represents the rate of ERTintervals per minute.

Decision module 340 compares an adaptive-rate sensor indicated rate(SIR) provided by an adaptive rate sensor and the calculated ERT rate.Decision module 340 then selects a maximum pacing rate (MPR) as shown inEquation 4:

MPR=minimum(SIR, ERT rate)  (4)

As Equation 4 indicates, the MPR will be determined by comparing the SIRand ERT rate and selecting the minimum, or slower, of the two.

If the adaptive-rate sensor indicated rate (SIR) is slower than the ERTrate, the SIR will be communicated to the pacemaker, as shown in module350. Otherwise, if the ERT rate is slower than the SIR, the ERT ratewill be communicated to the pacemaker, as shown in module 360.Therefore, through utilization of this methodology 300, a pacing ratelimiter may effectively limit the maximum pacing rate presented to apacemaker by comparing the SIR to the ERT rate and selecting the ERTrate if the SIR exceeds the ERT rate.

The methodology illustrated in FIG. 3 can be implemented in a pacingrate limiter as part of a CRM system. A CRM system 400 is illustrated inFIG. 4. Generally included are adaptive rate sensor 410, pacing ratelimiter 420 according to a preferred embodiment of the invention, and apacemaker 430. The adaptive rate sensor 410 may include any of theadaptive rate sensors as described above. Further, adaptive rate sensor410 may also include an intrinsic atrial rate as measured by electrodesplaced within the atria, as described above.

Adaptive rate sensor 410 is coupled to pacing rate limiter 420, andpacing rate limiter 420 is coupled to pacemaker 430. The adaptive ratesensor 410 may function to provide an adaptive-sensor indicated rate ifadaptive rate sensor 410 is an adaptive rate sensor, or an intrinsicatrial rate. Pacing rate limiter 420 then selects a maximum pacing ratebased on the minimum of an ERT rate calculated by the pacing ratelimiter and the rate provided by 410. The pacing rate limiter 420 thencommunicates the maximum pacing rate to the pacemaker 430. Pacemaker 430can then control the pacing stimulation frequency of electrical pulsescommunicated to the heart based on an escape interval as calculated fromthe maximum pacing rate.

It should be understood that the CRM system 400 as shown in FIG. 4 is byway of example only and that other configurations may be utilizedwithout departing from the spirit of the invention. For example, it ispossible that adaptive rate sensor 410, pacing rate limiter 420, andpacemaker 430 could all be implemented in a single unit or may beimplemented as separate units. Further, adaptive rate sensor 410 couldbe coupled directly to pacemaker 430, and selection of the maximumpacing rate could be conducted by pacemaker 430 based on the rateprovided by adaptive rate sensor 410 and the ERT rate from pacing ratelimiter 420. Other configurations are also possible.

A pacing rate limiter 500 according to a preferred embodiment of theinvention is shown in FIG. 5. The rate limiter 500 comprises a main unit520 with an input port 526 and an output port 528. The main unit 520 iscoupled to ER sensor 522 and T-wave sensor 524. Input port 526 isadapted to receive an adaptive-rate sensor indicated rate from anadaptive rate sensor, an intrinsic atrial rate, or other such pacingdevice. ER sensor 522 detects an evoked response and communicatesdetection of the evoked response to the main unit 520 of the pacing ratelimiter 500. Similarly, T-wave sensor 530 is adapted to detect andcommunicate detection of the T-wave to main unit 520.

Output port 540 can be coupled to a pacemaker or other such CRM devicefor the heart. The pacing rate limiter 500 may calculate and communicatea maximum pacing rate to the pacemaker by way of output port 500. Onceagain, it should be understood that the embodiment shown in FIG. 5 is byway of example only, and that other configurations for pacing ratelimiter 500 may be accomplished without departing from the invention. Byway of example, the main unit 520 and sensors 522 and 524 may beimplemented as a single unit or as separate units as shown. Further, thepacing rate limiter 500 may possibly be implemented as part of anadaptive rate sensor or as part of a CRM device.

The present invention should not be considered limited to the particularexamples described above, but rather should be understood to cover allaspects of the invention as fairly set out in the attached claims.Various modifications, equivalent processes, as well as numerousstructures to which the present invention may be applicable will bereadily apparent to those of skill in the art to which the presentinvention is directed upon review of the instant specification.

1. A cardiac rhythm management system, comprising: a pacemakerconfigured to pace a heart at a paced heart rate; and a pacing ratelimiter coupled to the pacemaker, the pacing rate limiter configured tocouple to an ER sensor to detect a paced ventricular evoked response andto couple to a T-wave sensor to detect a T-wave; wherein the cardiacrhythm management system is configured to calculate an ERT intervalbased on the paced ventricular evoked response and the T-wave; andwherein the cardiac rhythm management system is configured to provide amaximum pacing rate based on the ERT interval.
 2. The cardiac rhythmmanagement system of claim 1, wherein the cardiac rhythm managementsystem is configured to adjust the paced heart rate based on the maximumpacing rate.
 3. The cardiac rhythm management system of claim 1, whereinthe cardiac rhythm management system is configured to limit the pacedheart rate based on the maximum pacing rate.
 4. The cardiac rhythmmanagement system of claim 1, further comprising an adaptive rate sensormodule configured couple to an adaptive rate sensor and to measure anadaptive-rate sensor indicated rate, wherein the cardiac rhythmmanagement system is configured to calculate an ERT rate based on theERT interval, and wherein the cardiac rhythm management system isconfigured to compare the ERT rate to the adaptive-rate sensor indicatedrate to provide a maximum pacing rate.
 5. The cardiac rhythm managementsystem of claim 4, wherein the cardiac rhythm management system isconfigured to select the maximum pacing rate as a minimum of theadaptive-rate sensor indicated rate and the ERT rate.
 6. The cardiacrhythm management system of claim 4, wherein the adaptive rate sensormodule includes the adaptive rate sensor.
 7. The cardiac rhythmmanagement system of claim 6, wherein the adaptive rate sensor includesat least one of a minute ventilation sensor, a QT interval sensor, or anelectrode configured to provide an intrinsic atrial rate to the adaptiverate sensor module.
 8. The cardiac rhythm management system of claim 6,wherein the adaptive rate sensor includes an accelerometer basedactivity sensor.
 9. The cardiac rhythm management system of claim 6,wherein the adaptive rate sensor includes a transthoracic impedancebased minute ventilation sensor.
 10. The cardiac rhythm managementsystem of claim 1, wherein the pacing rate limiter is configured todetect a segment of the T-wave selected from the group consisting of anonset of the T-wave, a peak of the T-wave, and a conclusion of theT-wave.
 11. The cardiac rhythm management system of claim 1, wherein thecardiac rhythm management system is configured to average at least twoERT intervals to determine the ERT interval.
 12. The cardiac managementsystem of claim 1, wherein a single unit includes the pacemaker and thepacing rate limiter.
 13. A cardiac rhythm management system for a heart,the system comprising: a pacemaker configured to pace a heart at a pacedheart rate; a pacing rate limiter in communication with the pacemaker;an evoked response (ER) sensor in communication with the pacing ratelimiter, wherein the pacing rate limiter and the ER sensor areconfigured to detect a paced ventricular evoked response; a T sensor incommunication with the pacing rate limiter, wherein the the pacing ratelimiter and T sensor are configured to detect a T-wave; and an adaptiverate sensor in communication with at least one of the the pacing ratelimiter or the pacemaker, wherein the adaptive rate sensor is configuredto provide an adaptive-rate sensor indicated rate, wherein the pacingrate limiter includes a main unit configured to calculate an ERT ratebased on the detected paced ventricular evoked response and the detectedT-wave, and wherein the pacing rate limiter is configured to communicatea minimum of the ERT rate and the adaptive-rate sensor indicated rate tothe pacemaker as a maximum pacing rate.
 14. The cardiac rhythmmanagement system of claim 13, wherein the pacemaker is configured toadjust the paced heart rate based on the maximum pacing rate.
 15. Thecardiac rhythm management system of claim 13, wherein the pacemaker isconfigured to limit the paced heart rate based on the maximum pacingrate.
 16. The cardiac rhythm management system of claim 13, wherein theadaptive rate sensor includes at least one of an activity sensor, aminute ventilation sensor, a QT interval sensor, or an intrinsic atrialrate.
 17. The cardiac rhythm management system of claim 13, wherein theadaptive rate sensor includes an accelerometer based activity sensor.18. The cardiac rhythm management system of claim 13, wherein theadaptive rate sensor includes an transthoracic impedance based minuteventilation sensor.
 19. The cardiac rhythm management system of claim13, wherein the ER sensor utilizes small coupling capacitor technology.20. The cardiac rhythm management system of claim 13, wherein the pacingrate limiter is configured to detect the T-wave using one of an onset ofthe T-wave, a peak of the T-wave, or a conclusion of the T-wave.